This is an important part for your website but you can change it super easy way.
Go to Admin PanelThen click on Appearance buttonClick on Customize buttonClick Header
You can take header in several positions such as horizontal left, right, center etc as you want.You can keep your header in full width.You can set your header background style from here.You can set header text color from here.You can set your header transparent overlay light or dirk.You can set header sticky from here.You can set mobile break point from here.You can set mobile logo align from here.You can set mobile menu align from here.Set the menu style displayed in the mobile position.